A New Family of Barbarian Asteroids
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Downloaded from https://academic.oup.com/mnras/advance-article-abstract/doi/10.1093/mnras/stz451/5319141 by Washington University at St Louis user on 14 February 2019 Brang¨ane:a new family of Barbarian asteroids A. Cellino1, S. Bagnulo2, P. Tanga3, M. Devog`ele4, Ph. Bendjoya3, E. Reilly2, J.-P. Rivet3 and F. Spoto3;5 1 INAF - Osservatorio Astrofisico di Torino, Pino Torinese 10025, Italy. E-mail: [email protected] 2 Armagh Observatory, College Hill, Armagh BT61 9DG, UK. 3 Universit´ede la C^oted'Azur, Observatoire de la C^oted'Azur, CNRS, Laboratoire Lagrange, France 4 Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ 86001, USA 5 IMCCE, Observatoire de Paris, 77 av. Denfert-Rochereau, 75014 Paris, France Accepted After. Received Before; in original form: Earlier ABSTRACT The so-called Barbarian asteroids (from the name of the prototype of this class, as- teroid (234) Barbara) are extremely interesting objects because they might be the survivors of a very early generation of planetesimals. We have identified in the past the first case of an asteroid family (the Watsonia family) whose members are Barbar- ians, all issued from the collisional disruption of a sizeable parent body. Here we report on the identification of another family, called after the name of its biggest member, asteroid (606) Brang¨ane,consisting of objects displaying the polarimetric properties characterizing the Barbarian behaviour. This is the first recognized case of a swarm of small Barbarians issued from a quite recent cratering event. Key words: asteroids: polarization { asteroids: composition. 1 INTRODUCTION metric data, has been recently published by Devog`eleet al. (2018). These authors found conclusive evidence that all The discovery of an anomalous polarimetric behaviour of Barbarians, which had been previously classified as mem- (234) Barbara (Cellino et al. 2006) was the first recognition bers of different taxonomic classes based on their reflectance of the existence of a new class of asteroids (now commonly spectra at visible wavelengths, once the properties of both referred to as the Barbarians) which are very interesting the visible and near IR regions of the reflectance spectrum in many respects. Some of these objects are known to ex- are taken into account, appear to belong to a unique taxo- hibit anomalous reflectance spectra, characterized by fea- nomic class: the L class defined by DeMeo et al. (2009). tures that were interpreted by Burbine et al. (1992); Sun- Devog`eleet al. (2018) found that the spectra of L-class shine et al. (2008) as diagnostic of a very high content of objects can be successfully modeled using mixtures of the refractory compounds on their surfaces, including primar- fluffy kind of spinel present in CAIs, MgO-rich olivine and ily the aluminous spinel mineral (MgAl2O4). This is a ma- the mineral compounds found in the matrix of CV3 mete- jor component of the so-called Calcium Aluminum-rich In- orites. They also found that the presence of CAIs is respon- clusions (CAI) found in some meteorites. With their ages sible for the anomalous polarimetric inversion angle char- around 4.567 Gyrs, derived from isotopic abundances, CAIs acterizing the Barbarians (for an explanation of this and are the oldest samples of solid matter found in the Solar sys- other polarimetric parameters, see, for instance, Cellino et tem. As pointed out by Sunshine et al. (2008), CAIs are gen- al. 2015). In particular, they pointed out the presence of a erally classified, on the basis of petrography and geochem- variation of the inversion angle of asteroid (234) Barbara istry, into three major groups. Among them, the so-called as a function of wavelength, and interpreted it as a conse- ”fluffy type A" (FTA), characterized by a porous, strongly quence of the known wavelength-dependent variation of the non-compact structure, is found in all chondritic meteorites, refractive index of the spinel mineral. and produces a strong absorption feature in the reflectance Until a couple of years ago, it was not clear whether the spectrum, interpreted as a consequence of a high concentra- properties of Barbarians were determined by anomalous sur- tion of FeO-rich spinel. It is also believed that, as opposite to face properties, or by structural composition. In this respect, the other types of CAIs, FTAs may never have been totally a major step forward has been the discovery by means of po- melted by any transient heating event after their formation. larimetric measurements that the Watsonia asteroid family The most extensive analysis of objects belonging to the (Novakovi´cet al. 2011) consists of Barbarian objects (Cellino Barbarian class, based on both spectroscopic and polari- et al. 2014). Table 1. Total number of currently recognized members of the Downloaded from https://academic.oup.com/mnras/advance-article-abstract/doi/10.1093/mnras/stz451/5319141 by Washington University at St Louis user on 14 February 2019 Brang¨anefamily, and the lower and upper limits of the intervals covered by its members in the space of the proper orbital elements semi-major axis, eccentricity and sine of inclination. N. of proper semi-major proper sine of proper members axis (AU) eccentricity inclination 325 2.571 { 2.597 0.178 { 0.183 0.165 { 0.168 Figure 2. The same as Fig. 1, but for the family of Watsonia, the only one previously identified family of Barbarians (Cellino et al. 2014). As shown by the morphology of its cumulative size distribution, the Watsonia family may suggest a more energetic disruption event, rather than a typical cratering event as in the case of the Brang¨anefamily. been taken from the AstDys site1, where a regularly updated list of members of asteroid families is publicly available. The cumulative size distribution of the family is shown in Fig. 1. The sizes of the family members have been taken directly from the NEOWISE catalog (Mainzer at al. 2016), whenever available. The same albedo data have been also Figure 1. Cumulative size distribution of the Brang¨anefamily. used to derive the average albedo of the family, 0:116 ± The typical trend characterized by a large difference in size be- 0:005, very similar to the geometric albedo of Brang¨aneitself tween the largest member and the rest of the family members (0:113±0:021). The sizes of family members not observed by clearly shows that the family is the outcome of a cratering event. WISE were computed from the known relation linking the At sizes smaller than about 2 km the size distribution becomes size to the absolute magnitude H and the geometric albedo shallower, an indication of incompleteness of the recognized small- est members pV : log (D) = 3; 1236 − 0:2 H − 0:5 log (pV ) (1) where D is the equivalent diameter in km. In the compu- tations, the value of H was taken from the file listing the Asteroid families are groups of objects issued by the family members available in the AstDys site. For the value collisional disruption of single parent bodies. The fact that of the geometric albedo p we adopted the NEOWISE based the members of the Watsonia family are Barbarians strongly V average value of 0.116 mentioned above. suggests that the Barbarian properties are not purely surface The Brang¨anefamily is the likely outcome of a crater- effects, because the members of a same family derive from ing event. This is suggested by the large difference in size material located at different depths inside the parent body. between its largest member, (606) Brang¨ane,having an ab- This was also the first discovery of Barbarian asteroids of solute magnitude H = 10:22 and an estimated size of about relatively small sizes. 36 km (Mainzer at al. 2016), and the rest of the family mem- The main belt asteroid (606) Brang¨ane,about 40 km in bers. The latter have absolute magnitudes generally fainter size (Mainzer at al. 2016), belongs to the L-class (DeMeo et than 14, and sizes mostly below 5 km. In the cases of families al. 2009). It is the biggest member of another family (Milani issued from cratering events, the overall trend of the cumu- et al. 2014). Table 1 summarizes the main parameters of the lative size distribution tends to exhibit at the largest sizes Brang¨anefamily which, according to Spoto et al. (2015) is the result of a very recent cratering event (the age between 30 and 50 Myrs being derived by the analysis of the \V- shaped" diagram, see below) In this Table, the data have 1 http://hamilton.dm.unipi.it/astdys/index.php?pc=5 a characteristic concave trend (Tanga et al. 1999; Durda et Downloaded from https://academic.oup.com/mnras/advance-article-abstract/doi/10.1093/mnras/stz451/5319141 by Washington University at St Louis user on 14 February 2019 al. 2007). For a comparison, Fig. 2 shows the cumulative size dis- tribution of the Watsonia family. The morphology of the size distribution, computed in exactly the same way as in the case of Brang¨ane,as explained above, is clearly different with respect to the case of Brang¨ane,as it is not charac- terized by a big difference between the size of the largest member and those of the other biggest family members. It can also be noted that in the case of Watsonia there is a stronger depletion of objects at the low-end of the size dis- tribution. This can be partly due to observational bias, due to the fact that the perihelion distances of the Brangane family are lower than those of Watsonia members, due to a non-negligible difference in orbital eccentricity. On the other hand, the two families are located at fairly similar heliocen- tric distances (the orbital semi-major axis of (729) Watso- nia is about 2.76 AU) and, consisting of objects displaying similar spectroscopic and polarimetric properties, also the albedo of the members of the two families should be simi- lar.